Accumulations of static electrical charge can cause a variety of adverse effects. Discharges of static electricity are discomforting to people and can disrupt the operation of electronic equipment such as computers. Problems with static charge build-up have become particularly acute in certain industrial operations of which the manufacture of miniaturized solid state electronic components is a prominent example.
Discharges of static electricity can destroy the minute conductive paths in microchip wafers or the like. Charge accumulations on such wafers or the like also attract particulate contaminants which can cause the product to become defective.
Maintaining a high level of air ionization in the vicinity of objects which are to be protected is a highly effective technique for suppressing static charge build-up in clean rooms where electronic components are manufactured or at other locations. Charge accumulations on objects attract air ions of opposite polarity which then neutralize the charge.
Most air ionizing systems have one or more sharply pointed electrodes to which high voltage is applied. The resulting intense electrical field near the point of the electrode dissociates molecules of the constituent gases of air into positively and negatively charged ions. Ions having a polarity or charge opposite to that of the electrode are attracted to the electrode and neutralized. Ions of similar polarity are repelled by the electrode and by each other and disperse outwardly into the surrounding air. Ion movement from the electrode to the region of objects that are to be protected is usually accelerated by providing an air flow from the electrode to the object region.
Air ionizing systems intended for static charge suppression are usually designed to generate both positive and negative ions as the charges to be suppressed may be of either polarity. This may be accomplished by using two electrodes having opposite voltages or by periodically reversing the voltage on a single electrode. Production of both types of ion simultaneously tends to reduce the effective range of the apparatus as intermixed positive and negative ions rapidly neutralize each other by charge exchange.
Prior U.S. Pat. No. 4,542,434 of Scott J. S. Gehlke et al, issued Sept. 17, 1985 and entitled "Method and Apparatus for Sequenced Bipolar Air Ionization" (assigned to the assignee of the present application) describes a method and apparatus which extends the range of bipolar air ionizers and offers other advantages as well. In the system of that patent, timing signals initiate positive and negative ion generation at spaced apart electrodes during alternate time periods which are separated by off intervals during which no ion generation occurs. This allows an air flow to carry each pulse of ions a substantial distance away from the electrodes before significant intermixing and mutual neutralization of the two types of ions begins.
Precise control of the ion output rate is desirable in apparatus of the above described kind. Effective static charge suppression at a particular location requires that the ratio of positive to negative ions be within a narrow range of values and that the total concentration of ions in the air also be at or close to an optimum value. An excess of ions of one polarity can have the counter-productive effect of imparting charge to objects. A low concentration of ions may not adequately neutralize static charges and an overly high concentration may also have adverse effects. The optimum ratio of positive to negative ions and the optimum total ion concentration that are needed vary from location to location. The optimum ratio and concentration may also vary at a particular location over a period of time because of changes in activities, equipment, air flow patterns or other conditions at the location. The air ion content at the location can also depart from the desired levels because of changes in the ionizing apparatus itself such as electrode deterioration from corrosion, utility power line voltage fluctuations or other causes.
Thus the air ionizing apparatus should enable separate adjustment of the rates of generation of both positive and negative ions and the ion content of the air at the location should be monitored so that readjustments can be made when changed conditions make that advisable.
In the system of the above identified U.S. Pat. No. 4,542,434, positive and negative electrode voltages, the timing of periods of positive and negative ion generation and the duration of the off periods between periods of ion generation can each be independently adjusted. This enables tuning of the system to provide a ratio of positive to negative ions and a total ion concentration that is suited to the needs of the particular location where the system is installed. Ion levels at the site can then be monitored with sensing instruments and manual readjustments can be made when changed conditions make that necessary.
Bipolar air ionizers of the above discussed kind have at least one negative ion producing electrode coupled to a negative high voltage generator and at least one positive ion producing electrode connected to a positive high voltage generator. A control system produces cyclical timing signals that alternately actuate the negative and positive high voltage generators so that production of only one type of ion occurs at any given time. This avoids an immediate neutralization of ions which would otherwise occur from charge exchange between the ions of opposite polarity. The control system can also be adjusted to provide an off interval following each actuation of a high voltage generator during which neither generator is actuated. Ions of each polarity may then travel a substantial distance away from the electrodes before ions of the opposite polarity are produced. This extends the effective range of the air ionizing system by delaying the intermixing of ions of opposite polarity and thereby delaying neutralization by mutual charge exchange.
Heretofore, a characteristic of the high voltage generators has resulted in an undesirable prolongation of ion production following each ion generation period as called for by the timing system. Each high voltage generator contains capacitors which are charged up to a high voltage level each time the generator is actuated by the control system. A period of time is required for discharge of the capacitors after the control system has deactuated the high voltage generator. Consequently, ion production continues to occur for a limited period, at a diminishing rate, during the off intervals called for by the control system. This reduces precision of control of ion production rate and timing and reduces the effectiveness of the off intervals for extending the range of the air ionizer in the manner described above. The problem becomes particularly pronounced in situations where there is a low air flow rate in the vicinity of the ionizing electrodes as the residually produced ions are not quickly carried away from the electrode region.
Another problem can be encountered with such bipolar air ionizing systems under certain operating conditions. As previously pointed out, changes in the total ion output and/or the ratio of negative and positive ion output rates may be desirable during operation because of changes in activities, equipment, air flow patterns or other conditions at the location where static charges are to be suppressed. Monitoring of the ion content of the air and adjustment of negative and/or positive ion output can be done manually but it is often more advantageous to provide a feedback system which does this automatically and continually in response to signals from an air ion sensor.
The need for changes in ion output may vary at different locations in a room. Different rates of ion output, at the different locations can be provided for by using a plurality of spaced apart pairs of positive and negative air ionizers with each pair having its own individual ion sensor and feedback circuit. This degree of localized control is not needed in some installations and cost considerations, lack of space or other factors may dictate that a single sensor and feedback circuit be used control all or a group of such pairs of air ionizers. Prior feedback circuits, designed for individual control of a single air ionizer, are not ideally suited for joint control of a group of spaced apart units of the above described kind. The single sensor and feedback circuit responds immediately to momentarily fluctuations of ion content at one particular location in the room. These fluctuations are not necessarily indicative of conditions at other locations in the room. Consequently, the single sensor and feedback circuit may cause changes in the rate of ion production and/or the ratio of negative to positive ions at the other locations at times when such changes are not needed at the particular location.
The present invention is directed to overcoming one or more of the problems discussed above.